1. Field of the Invention
The present invention relates to floating type analog-to-digital converters capable of converting an analog signal to a digital signal in a wide dynamic range.
2. Description of the Related Art
FIG. 1 shows an example of a conventional floating type analog-to-digital converter (herein after referred to as "A/D converter"). As shown in FIG. 1, the floating type A/D converter includes two A/D converters 21, 22, and amplifiers 11, 12 are provided on the input sides of the respective A/D converters 21, 22, for amplifying an analog signal to be converted. The amplifier 11 has a predetermined gain A (A&gt;1), and the amplifier 12 has a gain that is equal to 1. With this arrangement, the A/D converter 21 receives an analog signal obtained by amplifying the analog signal to be converted by an amplification factor of A, and the A/D converter 22 receives the analog signal to be converted with its level being maintained.
The A/D converters 21 and 22 have the same structure, and each of these converters 21, 22 includes a .DELTA..SIGMA. modulator 101, and a decimation filter 102. The .DELTA..SIGMA. modulator 101 performs .DELTA..SIGMA. modulation on the analog signal supplied from the amplifier 11, 12, and generates or outputs bit stream data as a result of PDM (Pulse Density Modulation) in which the analog signal is equivalently modulated depending upon its level. The bit stream data is generated from the .DELTA..SIGMA. modulator 101 at a bit rate that is N times (for example, 64 times) the sampling frequency fs of the digital signal to be finally generated from this floating type A/D converter. The decimation filter 102 converts bit stream data generated from this .DELTA..SIGMA. modulator 101, intc a multiple-bit digital signal, while performing a subsampling operation thereon, and outputs the digital signal that corresponds to the sampling frequency fs.
As explained above, the A/D converter 22 receives the analog signal to be converted with its level being maintained or unchanged. As a result, the A/D converter 22 generates a digital signal that corresponds to the original analog signal. On the other hand, the A/D converter 21 receives an analog signal obtained by amplifying the original analog signal by an amplification factor of A. As a result, the A/D converter 21 generates a digital signal of a level that is A times that of the original analog signal, as long as the output digital signal is not clipped (saturated). In view of this, a digital signal processing device (DSP) 3 is provided on the output side of the A/D converter 21. The digital signal generated from the A/D converter 21 is multiplied by 1/A by means of this DSP 3, to thereby be modified into a digital signal that corresponds to the level of the original analog signal.
A switching detector 4 and a switching device 5 are provided, which serve to select one of the output signals of the DSP 3 and A/D converter 22, and generate the selected signal. Namely, the switching device 5 causes the switching detector 4 to monitor the output signal of each of the A/D converters 21, 22, and selects and generates the output signal of the DSP 3 when the output signal of the A/D converter 21 is not clipped, or selects and generates the output signal of the A/D converter 22 when the output signal of the A/D converter 21 is clipped.
In the arrangement as explained above, when the analog signal initially applied to the floating type A/D converter is weak, or has a low level or small amplitude, and the output signal of the A/D converter 21 is not clipped (or saturated), the amplifier 11, A/D converter 21 and the DSP 3 are used for performing A/D conversion with a resolution that is A times the inherent resolution of the A/D converter 21, so that the switching device 5 generates a digital signal obtained as a result of the high-resolution A/D conversion.
If the original analog signal has an increased level, and the A/D converter 21 receives this analog signal having an excessively high level or large amplitude, the output signal of the A/D converter 21 is clipped, whereby the output signal of the DSP 3 is also clipped. In this case, the switching device 5 outputs a digital signal into which the original analog signal was converted with the inherent resolution of the A/D converter 22.
Thus, the same analog signal is subjected to high-resolution A/D conversion and normal-resolution A/D conversion at the same time, and the high-resolution A/D conversion is automatically selected and the digital signal as a result of this conversion is generated when the original analog signal has a low level or small amplitude, while the normal-resolution A/D conversion is automatically selected and the digital signal as a result of this conversion is generated when the analog signal has a high level or large amplitude, thus enabling the A/D converter to perform A/D conversion in a wide dynamic range.
As explained above, the floating type A/D converter is capable of converting an analog signal into a digital signal in a wide dynamic range. However, the inventor of the present invention found in his studies that this floating type A/D converter has unstable frequency characteristics in a high frequency range. Details of this problem will be described below.
In an experiment, the inventor applied two impulses having the same waveform to the floating type A/D converter at different times, and obtained sample data of each impulse response waveform generated from the floating type A/D converter. In FIG. 2, plots or points "X" represent sample data of each impulse response waveform taken from the floating type A/D converter, and solid line B and dashed line C represent impulse response waveforms formed by connecting adjacent ones of the plots corresponding to each impulse. Although the impulse response waveform C lacks a peak as noted in FIG. 2, this is because sampling timing of the sample data is shifted from a peak point of the impulse response, and does not mean that the original impulse response waveform does not have a peak.
Then, the impulse response waveforms B and C thus obtained were processed according to FFT (fast Fourier transform), and a frequency characteristic of the gain of the floating type A/D converter was derived from each of the impulse response waveforms B and C. In FIG. 3, D represents a frequency characteristic derived from the impulse response waveform B, and E represents a frequency characteristic derived from the impulse response waveform C. When comparing the frequency characteristics D and E with each other, it is found that these characteristics greatly differ in a high frequency range.
The frequency characteristics D and E were measured at different times, but using impulses having the same waveform. Accordingly, these frequency characteristics D and E should be identical with each other. However, a high-frequency portion of the frequency characteristic derived from the impulse response waveform changes with a slight shift or change of the input timing of the impulse. Thus, the floating type A/D converter has an unstable frequency characteristic in a high-frequency region, as stated above.
The inventor of the present invention made repeated attempts to find out the reason why the frequency characteristic is unstable and unrepeatable in a high-frequency region, based on various hypotheses, but it was extremely difficult to find the reason. Then, the inventor conducted the same experiment as described above, using a successive approximation type A/D converter, rather than the floating type A/D converter. The result of this experiment revealed a cause of occurrence of the unstable, unrepeatable frequency characteristic in a high-frequency region in the floating type A/D converter. This will be described in greater detail below.
Referring to FIG. 4, impulse response waveforms F and G shown in this figure were obtained by applying impulses to the successive approximation type A/D converter at different times, in the same manner as described above. In FIG. 5, frequency characteristics H and I were obtained by processing sample data of the respective impulse waveforms F and G according to FFT (fast Fourier transform). It is apparent from FIG. 5 that the frequency characteristics H and I are approximate or very close to each other. Namely, the successive approximation type A/D converter does not suffer from the problem of unstable frequency characteristic in a high-frequency region, which occurs only in the floating type A/D converter.
Since the frequency characteristics of the successive approximation type A/D converter are different from those of the floating type A/D converter as indicated above, there must be any difference in the impulse response waveforms between these two types of converters. Under this assumption, the present inventor superimposed the impulse response waveforms (FIG. 2) obtained from the floating type A/D converter on the impulse response waveforms (FIG. 4) obtained from the successive approximation type A/D converter, and found that there was a significant difference between the corresponding impulse response waveforms obtained from these A/D converters. Namely, while each impulse waveform has the form in which pre-echo and post-echo are added to the leading and trailing ends, respectively, of a portion of the waveform that corresponds to the original impulse, the ratio of the amplitude of the pre-echo to that of the post-echo which appear in the floating type A/D converter is different from that which appear in the successive approximation type A/D converter.
In the case of the successive approximation type A/D converter, the amplitude of the pre-echo that precedes a peak portion of the waveform is not so different from that of the post-echo that follows the peak portion, as shown in FIG. 4. In the case of the floating type A/D converter, on the other hand, the amplitude of the pre-echo is significantly smaller than that of the post-echo, and the signal pattern of the pre-echo is asymmetrical with that of the post-echo, as shown in FIG. 2.
Between the impulse response waveforms of the floating type A/D converter and those of the successive approximation type A/D converter, there cannot be found any other characteristic difference than the difference as to whether the pattern of the pre-echoes is symmetrical or asymmetrical with respect to that of the post-echo. It may be therefore considered that the reason why the asymmetric pre-echo and post-echo patterns appear in the floating type A/D converter is related to the reason why the frequency characteristic of this type of A/D converter become unstable in a high-frequency region.
It will be now considered why the pre-echo and the post-echo have an asymmetric relationship in the floating type A/D converter. In the floating type A/D converter, the pre-echo and the post-echo are supposed to occur in the decimation filter as shown in FIG. 1. Since this decimation filter consists of a FIR filter having a linear phase characteristic, symmetric pre-echo and post-echo should be added to the input impulse. In the actual floating type A/D converter, however, the pre-echo and post-echo patterns of the impulse response waveform have an asymmetric relationship, as described above. The inventor of the present invention assumed that this asymmetry between the pre-echo and the post-echo may be caused by switching between the high-resolution A/D conversion and the normal-resolution A/D conversion as described above.
Referring to FIGS. 6A-6F and FIG. 1, the cause of the asymmetric relationship between the pre-echo and the post-echo according to the inventor's assumption will be now explained. Suppose an impulse as illustrated in FIG. 6A is applied to the floating type A/D converter constructed as shown in FIG. 1. In this case, an impulse of a level that is A times the level of the applied impulse is supplied to the A/D converter 21, as shown in FIG. 6B, while the applied impulse is supplied as it is to the A/D converter 22 with its level being maintained.
These impulses of FIG. 6A and 6B are converted by the respective A/D converters 21 and 22, into corresponding digital signals. In this A/D conversion, sample data of impulse response waveforms as illustrated in FIGS. 6D and 6E are generated from the respective decimation filters of the A/D converters 21 and 22. As shown in FIGS. 6D and 6E, symmetric pre-echo and post-echo are added to each of the impulse response waveforms that occur in the A/D converters.
In the meantime, the level of the output digital signal of each of the A/D converters 21 and 22 is limited to that which is equal to or lower than the maximum value (clipping level) that is determined depending upon a bit width of the corresponding converter. In the example of FIG. 6, the digital signal generated from the decimation filter of the A/D converter 21 exceeds a clip level at a point of time TX, as shown in FIG. 6D, and therefore the output digital signal of the A/D converter 21 is clipped at and after this point of time TX.
Accordingly, the switching device 5 selects the digital signal whose level is controlled by the DSP 3 to be 1/A times that of the output digital signal of the A/D converter 21, during a period before the point of time TX, and selects the output digital signal of the A/D converter 22 during a period after the point of time TX. As a result, the impulse response waveform generated from the switching device 5 has the form as shown in FIG. 6F, in which a portion of the impulse response waveform obtained from the DSP 3 which corresponds to the pre-echo or precedes the time TX is connected with a portion of the impulse response waveform obtained from the A/D converter 22 which follows the pre-echo or follows the time TX. In this case, the portion of the impulse response waveform (FIG. 6E) generated from the A/D converter 22 which corresponds to the post-echo is generated from the switching device 5 while maintaining its level, whereas the portion of the impulse response waveform generated from the DSP 3 which corresponds to the pre-echo has its level reduced by 1/A, and is thus generated from the switching device 5. As a result, the impulse response waveform generated from the switching device 5 takes the form as illustrated in FIG. 6F, in which the magnitude of the pre-echo is reduced as compared with that of the post-echo, and the pre-echo and post-echo included in the impulse response waveform are asymmetrical with each other. Thus, the asymmetry of the pre-echo and post-echo patterns occurs in the floating type A/D converter, according to the inventor's analysis.
In the known floating type A/D converter, it may be considered that its frequency characteristics become unstable in a high-frequency region because discontinuity or non-uniformity occurs between the portion of the impulse response waveform corresponding to the pre-echo, and the portion of the waveform that follows the pre-echo, for the reason as described above.